The invention relates to a process for the processing of fluids containing suspended particles. More specifically it relates to the elimination of particles suspended in gases or liquids and specifically to the elimination of very fine particles, whose dimensions can be as small as 0.01 .mu.m.
When the fluid is a gas, said particles can be constituted by aerosols, fine powders, ash, etc. In particular, gases containing suspended particles of this type are found in the internal atmospheres of nuclear or non-nuclear installations, e.g. in reactors, plants and laboratories, in installations for incinerating radioactive or non-radioactive waste, in liquid drying installations and all dust-containing installations.
When the fluid is a liquid, the particles can be constituted by insoluble matter, such as fine powders, e.g. powders of metallic oxides, colloidal particles, etc.
Examples of liquids containing particles of this type in suspension are liquid effluents and waste materials produced in nuclear power stations, nuclear plants and numerous industrial installations.
The presently most widely used process for processing fluid of this type consists of filtering them by passing through filtering elements having sufficiently fine mesh sizes to hold back the particles. However, with such a procedure, there is a progressive clogging of the filter, which leads to a variation in the operating characteristics of the filtering circuit and requires the periodic replacement of clogged filtering elements or the putting into operation of a countercurrent declogging stage by injecting pressurized fluid.
The replacement of the filters constitutes a serious disadvantage, particularly when the filtered particles are radioactive. Thus, due to the radioactivity deposited on these filters, safety problems are caused in the handling thereof for the purpose of obviating contamination and irradiation risks. Moreover, they must be conditioned in tight matrixes, such as of concrete, bitumen or thermosetting resins to enable the long-term storage thereof, which has the effect of diluting the radioactive dust in an inert material of very significant volume and thus increasing the volume of the waste. The variant consisting of declogging the filters by injecting pressurized fluid makes it possible to obviate this disadvantage, but is complex and difficult to perform.
To obviate this situation, consideration has been given to the treatment of the fluids by using cyclone separators or electrostatic filters, in order to eliminate the particles contained therein, but such devices have an inadequate stopping power with respect to such particles.
The present invention specifically relates to a process for the processing or treatment of fluids containing suspended particles and which obviates the aforementioned disadvantages.
According to the invention, the fluid to be processed undergoes a processing cycle comprising a filtering stage in order to extract part of the fluid in the purified state and a separation stage for extracting part of the particles present in suspension in the fluid, the processing cycle being started either by the filtering stage, or by the separating stage, and subjecting the particle-depleted fluid from the separating stage or the particle-enriched fluid from the filtering stage to a further processing cycle after adding thereto fluid to be processed.
For the performance of the processing or treatment cycle, the choice of the first stage is more particularly dependent on the particle concentration of the fluid to be processed.
Thus, if the fluid to be processed has a high particle concentration, preference is given to starting with the separating stage. In this case, the process consists of subjecting said fluid to a processing cycle comprising the following stages :
(a) a separating part of the particle present in the fluid to be processed in order to reduce the number of particles therein,
(b) circulating the resulting fluid in the first compartment of a filtering apparatus subdivided into first and second compartments by at least one porous, permeable wall having pores with dimensions smaller than those of the particles, so that at the outlet from said first compartment particle-enriched fluid is obtained and purified fluid is diffused into the second compartment,
(c) recovering the purified fluid which has diffused into the second compartment and
(d) recycling the particle-enriched fluid leaving the first compartment in order to subject it to a further processing cycle with the fluid to be processed.
Conversely, when the fluid to be processed has a relatively small particle concentration, preference is given to starting the processing cycle with the filtering stage. In this case, the process consists of subjecting the fluid to a processing cycle comprising the following stages:
(a') circulating the fluid to be processed in the first compartment of a filtering apparatus subdivided into first and second compartments by at least one porous, permeable wall having pores of sizes smaller than those of the particles, in order to obtain at the outlet of said first compartment particle-enriched fluid and purified fluid is diffused into the second compartment,
(b') recovering the purified fluid which has diffused into the second compartment,
(c') separating part of the particle present in the fluid leaving the first compartment in order to bring about particle depletion thereof and
(d') recycling the thus depleted fluid in order to subject it to a further processing cycle with the fluid to be processed.
In these two embodiments of the inventive process, the porous, permeable walls of the filtering apparatus are generally constituted by tubes within which is circulated the fluid to undergo particle enrichment. Thus, these tubes internally define the first compartment of the filtering apparatus.
Through the performance of the aforementioned processing or treatment cycle, the inventive process offers numerous advantages in connection with the processing of liquids and gases containing suspended particles.
Thus, through successively performing a particle enrichment stage and a particle depletion stage makes it possible to avoid clogging of the porous walls, by maintaining at an appropriate value the particle concentration of the fluid circulating in the filtering apparatus.
Moreover, said process makes it possible to obtain very advanced filtering, because it is merely necessary to choose the characteristics of the porous, permeable walls in order to obtain the desired filtering action.
Thus, the disadvantages of the prior art processes are avoided, i.e. the replacement of the filtering elements and the need to condition then in inert matrixes when they have been used for the treatment of fluids containing radioactive or toxic suspended particles. Moreover the volume of the radioactive or toxic waste to be treated is limited, because said waste is recovered in the form of a particle concentrate during the separating stage.
Furthermore, through preventing the clogging of the filtering apparatus, it is possible to carry out several processing cycles with stable operating characteristics for long periods.
The invention also relates to an apparatus for performing this process and which comprises:
a processing circuit successively comprising a filtering apparatus subdivided into first and second compartments by means of at least one porous, permeable wall having pores with sizes smaller than those of the particles, a particle separating device connected to the two ends of the first compartment of the filtering apparatus and means for permitting the circulation in the processing circuit of the fluid to be processed, PA0 means for introducing the fluid to be processed into the processing circuit, PA0 means for extracting a fluid which has diffused into the second compartment of the filtering apparatus and PA0 means for collecting the particles separated in the separating device.
In this apparatus, the means for circulating the fluid to be treated in the treatment circuit are arranged with respect to the means for introducing the fluid to be treated in such a way that the fluid successively circulates in the filtering apparatus and in the particle separating device or vice-versa.
Advantageously, the porous, permeable walls of the filtering apparatus are constituted by tubes, which internally define the first compartment. However, it is also possible to use other types of walls, e.g. porous plates.
The filtering apparatus used in the invention can be constituted by an ultrafiltration module having a plurality of porous tubes arranged parallel to one another, as in the modules used for the concentration and separation of the constituents present in a liquid.
The particle separating device can be constituted by a conventional device, e.g. an electrostatic filter, a dust separator operated by an impact or baffle action, a decanter, etc. Preferably, in the inventive process, use is made of a cyclone separator or a hydrocyclone, i.e. purely static apparatus using centrifugal force for extracting a particle-enriched fraction from a gaseous or liquid stream.